Air carrier Captain reported cockpit clocks jumped forward by 2.5 hours when in international airspace subject to GPS spoofing/interference. Attempts to contact dispatch via SAT COM and HF were unsuccessful during the 15 minutes of erroneous indications.

Narrative

As we approached KARIL intersection to cross from LRBB FIR to LHCC FIR we noticed both left and right clocks jumped ahead 2.5 hours from actual Zulu time. We had complied with position reporting at RODAR intersection hours before. The GPS NAV had been selected OFF; VOR/DME NAV had been selected ON; and the Tuning Control Panel (TCP) GPWS TERR OVRD had been selected OVRD at the time of the occurrence; having been set this way since RODAR. I sent Dispatch a message detailing the issue with the Clocks and said I would contact via SAT COM. SAT COM would not connect after multiple attemps. Dispatch sent an ACARS message to us that they got a busy signal when they tried to contact us.` I was in the process of setting up for an HF phone patch to Dispatch when the Clock times returned to normal. I then sent Dispatch an ACARS message to let them know our Clocks were operating correctly again. Since we had crossed the FIR boundary where we did not expect further GPS interference; we returned the GPS NAV; VOR/DME NAV; and TCP GPWS TERR OVRD to normal settings. We experienced no further interference. I would estimate that our clocks were in error for about 15 minutes total.